Tablet composition for the in-situ generation of chlorine dioxide for use in antimicrobial applications

ABSTRACT

A solid composition in the form of a tablet that generates and releases a biocidal solution comprising at least chlorine dioxide with an enhanced weight percent yield is presented. The composition comprises reactants capable of in-situ generation of chlorine dioxide comprising a chlorite donor that is coated with a gel-forming material that slows the rate of dissolution of the high solubility chlorite donor, a free halogen donor, and an acid source, resulting in an unexpectedly high weight percent yield and providing a controlled release of biocidal solution. The compositions of the invention show improved environmental stability which can reduce the cost of packaging and significantly increase the utility of the composition. The controlled release allows the use in multi-tablet chemical dispensers which may otherwise induce potentially explosive conditions or allow rapid release of the biocidal solution thereby inducing a spike in chemical concentration rather than a controlled and sustained release.

RELATED APPLICATIONS

This application is a Continuation-in-Part (CIP) of U.S. patentapplication Ser. No. 12/802,230 filed on 2 Jun. 2010, which is a CIP ofU.S. patent application Ser. No. 12/660,470 filed on 25 Feb. 2010, whichis a CIP of U.S. patent application Ser. No. 12/655,953 filed on 11 Jan.2010, which is a CIP of U.S. patent application Ser. No. 12/653,984filed on 22 Dec. 2009, which is a CIP of U.S. patent application Ser.No. 12/380,329 filed on 26 Feb. 2009, which is a CIP of U.S. patentapplication Ser. No. 11/253,977 filed on 18 Oct. 2005, which is now U.S.Pat. No. 7,534,368, which is a CIP of U.S. patent application Ser. No.11/154,086 filed on 15 Jun. 2005 which is now U.S. Pat. No. 7,514,019,which is a CIP of U.S. patent application Ser. No. 11/070,132 filed on 1Mar. 2005 which is now Abandoned. The entire contents of these patentapplications are incorporated by reference herein.

FIELD OF INVENTION

This invention relates to a solid composition in the form of a tabletthat produces chlorine dioxide when contacted with an aqueous solutionthat is sufficiently stable for bulk packaging, provides a controlledrelease of at least chlorine dioxide, and is suitable for use in amulti-tablet chemical dispenser. The tablet of the invention provides atleast a 20 weight percent yield of chlorine dioxide and allows in excessof 30 weight percent yield of chlorine dioxide. The weight percent yieldachieved by a solid composition in the form of a tablet of the inventionis unprecedented.

BACKGROUND

Oxidizing biocides are commonly used for the treatment of recirculatingsystems such as industrial cooling systems. It is common for tabletforms to be applied thru feeders such as flow through chlorinators orbrominators. However, in many instances chlorine and bromine alone arenot sufficient for the control of microbiological activity, especiallyin contaminated systems and/or where the pH is elevated which reducesthe effectiveness of chlorine and bromine oxidizers.

Chlorine dioxide is an effective antimicrobial agent for use in foodprocessing applications. Examples of food process applications includebut are not limited to: vegetable and fruit washing, cleaning of animalprocessing equipment, cleaning of animal carcasses, treatment of poultryand animal habitats.

Chlorine dioxide has been shown to be very effective for the control ofmicrobiological organisms. However, cost effective generation ofchlorine dioxide requires on-sight generation from liquid reagents andsubstantial capital investment.

In recent years, tablets that generate chlorine dioxide have beendeveloped, however their use in the treatment of recirculating systemsis very limited due to high use cost and limited utility. High use costis attributed to the tablet's low yields of chlorine dioxide and poorenvironmental stability that requires costly individual packaging of thetablets. Also, the high reactivity and rapid release of the chlorinedioxide results in a spike of treatment rather than the desirablecontrolled release to achieve a sustained concentration of treatment,and subsequent potential for generation of explosive conditions whenapplied in multi-tablet chemical dispensers due to elevated levels ofpotentially hazardous and explosive gas.

U.S. Pat. No. 6,699,404 to Speronello (“the Speronello patent”)discloses a massive body having a porous structure which substantiallyincreases the percent conversion of chlorite to chlorine dioxide whencompared to chlorite powder. The Speronello patent discloses two typesof massive bodies: a water soluble type and a substantially waterinsoluble type. The substantially water insoluble massive body forms aporous framework that provides a higher efficiency of the conversioncompared to the water-soluble massive body. According to the test dataprovided in the Speronello patent the maximum concentration of chlorinedioxide produced by a massive body that forms the porous framework is149.4 mg/L. The water-soluble massive body reported (example 4) amaximum 27.4 mg/L.

In order to achieve 70% or more conversion of the chlorite to chlorinedioxide using the method disclosed in the Speronello patent, asubstantial amount of inert materials are added to produce the porousstructure or the porous framework. The level of inert salts ranges from18 wt. % to 80 wt. %, with higher weight percentages increasing theconversion efficiency. The high levels of inert material, particularlyin the water-soluble massive body, are further illustrated in commercialpractice. For example, Aseptrol®, which is the commercialized productembodying the invention disclosed in the Speronello patent, is a watersoluble tablet that requires 1.5 grams of Aseptrol® to 1 liter of waterto produce 100 mg/L chlorine dioxide. This equates to approximately 67mg/L chlorine dioxide based on 1 gram tablet per liter. The weight-%yield, which is defined as weight of the chlorine dioxide divided by theweight of the tablet, is low because of the high level of inertmaterial. According to the data reported in the Speronello patent, theweight % yield is less than 15 wt. %, and less than 3% in the case ofthe water-soluble massive body. Based on the commercial productAseptrol®, the weight percent yield of the water soluble commercialproduct is 6.7 wt. %.

It is desirable to increase the concentration of chlorine dioxideproduced by a given mass of tablet to improve the economics based on thecost per pound of the tablet material versus pounds of chlorine dioxideproduced. Such increase would also result in an overall performanceenhancement offered by higher concentrations of chlorine dioxide. Toachieve this objective, tablet conversion efficiency of >70% and a highreactant weight percent are desirable. It is also desirable tosubstantially increase the concentration of chlorine dioxide using acompletely water-soluble composition to eliminate the problemsassociated with water insoluble constituents or byproducts such asresidue silica based clays, or mineral salts such as calcium sulfate.

U.S. Pat. Nos. 6,384,006 and 6,319,888 to Wei et al. (“the Wei patents”)disclose a system for forming and releasing an aqueous peracid solution.The system includes a container and a peracid forming composition thatincludes about 10-60 wt. % of a chemical heater that, upon contact withwater, generates heat to increase the yield of the peracid.

The Wei patents describe the potential use of a viscosity modifierwithin a permeable container to increase the viscosity in the localizedarea from about 300 to about 2,000 centipoise. The increased viscosityrestricts and slows down the movement of peracid precursor and/orperoxygen source out of the permeable container. This results in anincreased residence time of the peracid precursor and peroxygen sourcewithin the permeable container, which in turn translates to a greaterreaction rate.

U.S. Pat. No. 6,569,353 to Giletto et al. (“the Giletto patent”)discloses using silica gel to increase the viscosity of various oxidantsincluding an in-situ generated oxidant in order to keep them in intimatecontact with the agents targeted for oxidation.

U.S. Published Application No. 2001/0012504 to Thangaraj et al. (“theThangaraj application”) discloses a composition for producing chlorinedioxide comprising an acid source and a chlorite source, and a methodcomprising enclosing the composition in a gelatin capsule or membranesheet such as a “tea bag”.

U.S. Pat. No. 5,688,515 discloses a composition comprisingtrichloroisocyanuric acid, sodium bromide, and dimethylhydantoin toproduce hypobromous acid.

Patent Application WO 2007/078838 discloses a composition comprising anoxidizer and bromide donor along with a chlorite donor to producechlorine dioxide. The compositions disclosed generate chlorine dioxiderapidly and preferably without the use of chlorine donors such aschlorinated isocyanurates. The compositions also require specialpackaging to prevent chlorine dioxide generation resulting from relativehumidity.

In order to improve reaction kinetics, the above references teach usingsubstantial quantities of inert materials to either provide a porousstructure as in the case of the Speronello patent, or heat as in thecases of the Wei patents. While viscosity modifiers are referenced inthe Wei patents, the viscosity range disclosed in the Wei patents doesnot reflect the formation of a gel.

U.S. Pat. No. 7,514,019 B2 discloses a solvent-activated reactorincluding a gel layer that allows for a water-soluble tablet compositionthat delivers at least a 70 wt % conversion of chlorite to chlorinedioxide and at least 14 wt % yield. However, the maximum yield ofchlorine dioxide achieved in the disclosed data was 18.1 weight percent.

U.S. Pat. No. 7,465,410 B2 discloses a solvent-activated reactorcomprising a core of reactants that are encapsulated by asolvent-permeable reactor wall. The solvent activated reactor allows fora convenient means of generating a target product, however provides noimprovements in weight percent yield or environmental stability thanthat disclosed in U.S. Pat. No. 7,514,019 B2.

U.S. Pat. No. 7,150,854 discloses a device comprising a substrate andreagents that permits the rapid release of relatively small quantitiesof chlorine dioxide in liquid water as needed and is therefore quiteuseful for sterilizing water such that it is potable and useful as agermicidal liquid. Furthermore, the present invention lends itself tothe separation of the reaction precursors into discrete zones ordomains, thereby resulting in increased shelf life and the avoidance ofexpensive packaging.

Search still continues for a method of stabilizing reactive componentsfor storage without compromising or limiting their function duringusage. Furthermore, it is highly desirable to have an environmentallystable tablet composition that provides an enhanced weight percent yieldof chlorine dioxide that is at least 20 wt %, preferably 25 wt % andmost preferably at least 30 wt %. Further still, the application ofchlorine dioxide is severely limited due to poor environmental stabilitywhich lends itself to individually wrapped packaging, increased usecost, and lack of controlled release.

SUMMARY

It has been discovered that the weight percent yield of chlorine dioxideresulting from a solid composition in the form of a tablet issurprisingly and substantially increased to levels never before possibleallowing at least 20 wt % yield, more preferably at least 25 wt % yield,and most desirably at least 30 wt % yield. Another benefit resultingfrom the disclosed invention is the dramatic improvements inenvironmental stability and the subsequent increased utility resultingfrom it such as bulk packaging and application of the composition in amulti-tablet dispenser. The invention provides for a tablet withenhanced weight percent yield of chlorine dioxide whether used alone orin combination with multiple tablets and a controlled release ofchlorine dioxide that can be optimized by altering the gelling agentchemistry.

In one aspect, the invention is a solid composition in the form of atablet with an enhanced weight percent yield that generates chlorinedioxide and releases an antimicrobial solution. The compositioncomprises reactants capable of generating the target product comprisingat least chlorine dioxide through a chemical reaction, and a gel-formingmaterial that allows for high yield and increased conversion of chloriteto chlorine dioxide. The chemical reaction is triggered when thereactants comprising the solid composition is contacted by an aqueoussolution. The reactants include a free halogen donor, a chlorite donor,an acid source, and a gel-forming material that makes up about 0.1 to 30weight % of the composition. All weight percents are based on the weightof the composition unless otherwise stated. The chlorite donor is coatedor encapsulated with at least some portion of the gel-forming material.Upon being exposed to the main solvent, the gel-forming material swellsforming a gelatinous structure that encapsulates the high solubilitychlorite donor thereby slowing the dissolution rate of the chlorite, andcreates a chamber within the composition enclosing some of the reactantssuch that the target product is generated in the chamber, wherein thegelatinous structure restricts diffusion of the reactants and the targetproduct out of the chamber, restricts the diffusion of the main solventinto the chamber, and wherein the gelatinous structure dissipates when adepletion level is reached inside the chamber. Different parts of thecomposition are exposed to the main solvent at different times.

The gelatinous structure functions as a temporary membrane thatrestricts the diffusion of the main solvent (aqueous solution) to thechlorite thereby retaining chlorite in contact with the other reactants,restricts the dissolution of reactants, allows formation of in-situgenerated chlorine dioxide by reaction between high concentrations ofreactants thereby inducing high percentage of conversion of chloriteanion to chlorine dioxide, and restricts the dissolution of the chlorinedioxide into the bulk of the main solvent. As a result of this theorizedmechanism, a high conversion of chlorite anion to chlorine dioxideoccurs allowing at least 70 wt % conversion, more preferably 80%conversion and most preferred 90% conversion of chlorite anion tochlorine dioxide. Of great benefit and value is the ability of a solidcomposition in the form of a tablet to generate chlorine dioxide havinga weight percent yield (wt % yield) of at least 20 wt %, and morepreferably at least 25 wt % and most preferred at least 30 wt %.

In another aspect, the invention is a solid composition in the form of atablet that generates chlorine dioxide and releases an antimicrobialsolution for the treatment of food processing applications include butare not limited to: vegetable and fruit washing, cleaning of animalprocessing equipment, cleaning of animal carcasses, treatment of poultryand animal habitats. The tablet composition has substantialenvironmental stability.

In another aspect, the invention is a solid composition in the form of atablet that generates chlorine dioxide and releases an antimicrobialsolution for the treatment of recirculating systems including industrialcooling systems, swimming pools, spas, fountains, water parks; and hardsurfaces such as those located in buildings and institutions such ashospitals, schools, office buildings, military bases and the like. Theinvention is also suitable for sanitizing surgical instruments andequipment exemplified by scalpels and endoscopes.

In another aspect, the invention is a solid composition in the form of atablet that generates chlorine dioxide and releases an antimicrobialsolution for the treatment of emergency drinking water. Emergencydrinking water may be used by hikers, campers, survivalist, military,and emergency services such as FEMA.

In another aspect, the invention provides for solid composition in theform of a tablet capable of achieving conversion of chlorite anion tochlorine dioxide of at least 70 wt %, with a preferred conversion ofleast 80 wt %, and a most preferred conversion of at least 90 wt %. Theconversion of chlorite anion to chlorine dioxide can be achieved using asubstantially water soluble composition that provides at least 20 wt %yield of chlorine dioxide, more preferably the composition can beformulated to achieve at least 25 wt % yield of chlorine dioxide, andmost preferably the composition can be formulated to achieve at least 30wt % yield of chlorine dioxide.

In another aspect, the invention is a solid composition in the form of atablet that has increased environmental stability. The proper selectionand application of the gel-forming material can make the compositionextremely stable until it is in intimate contact with an aqueoussolution. Even when immersed in an aqueous solution, the said tablet canbe made to have a delayed reaction because of the formation of a viscousfilm that restricts the water and movement of the dissolving reactants.It will be shown that this restriction can also result in aself-limiting tablet, such that the generation of chlorine dioxide canbe made to substantially slow or stop when the weight ratio of thetablet to water gets too high. It is believed the increasing viscosityelevates the concentration of the reactants to where they reach theirsaturation level and the tablet slows its dissolution rate.

In another aspect, the invention is a composition that releases at leastchlorine dioxide at a controlled rate thereby providing an antimicrobialsolution for an extended period of time and allowing use in multi-tabletchemical dispensers. Tablets can be designed to release theantimicrobial solution over minutes, hours or days instead of a rapidrelease of chlorine dioxide like compositions disclosed in the referenceprior art, which in an enclosed compartment of a chemical dispenser canproduce potentially catastrophic conditions. The ability to customizethe release rate of chlorine dioxide expands the utility of the solidcompositions of the invention. Tablets can be made to generate a high wt% yield of chlorine dioxide quickly by applying a single or multipletablets to a volume of water to produce an antimicrobial solution fordisinfecting surfaces. In another example, a plurality of tablets can beadded to a multi-tablet dispenser that serves to produce anantimicrobial solution and apply said solution to a recirculating systemsuch as a cooling system for an extended period of time. The ability toalter the gel chemistry of the tablet greatly expands the safety andutility of the tablets.

In another aspect, the invention is a method of producing a compositionthat generates chlorine dioxide with enhanced weight percent yield andenvironmental stability. The method entails coating the chlorite donorwith a gel-forming material comprising a polymer, more preferably andsynthetic polymer that is an oxidation resistant polymer. Thegel-forming material slows the dissolution of the high solubilitychlorite. The reactants of the composition are combined, mixed andagglomerated into a tablet composition. When the tablet composition isimmersed in water, the coated chlorite forms a viscous gel at thesurface of the high solubility chlorite, thereby restricting itsdissolution. The gel from a plurality of polymer coated chloriteinterconnect and form chambers that entrap the other reactants in thecomposition thereby enhances the contact time between the free halogendonor exemplified by trichloroisocyanuric acid and acid sourceexemplified by fumaric acid. The gel forms a temporary membrane thatfunctions as a reactor by keeping the reactants in intimate contactuntil a high conversion of chlorite anions to chlorine dioxide takesplace. It is also desirable, but not required to use a low solubilityacid source such as fumaric acid.

In yet another aspect, by combining low solubility free halogen donorwith the encapsulated chlorite donor that has a restricted dissolutionrate results in unexpected dramatic increases in the tablet stabilityand weight percent yield that exceed 25 wt % and most preferred exceeds30 wt %.

The resulting tablets composition has improved environmental stabilityand can be made suitable for bulk packaging. Current tablets thatgenerate relatively low weight percent yield of chlorine dioxide arepackaged as individual tablets to protect from relative humidity andpremature release of chlorine dioxide. The tablets of the invention aresufficiently stable as to make them suitable for bulk packaging whereinmultiple tablets are provided in one package. This substantially reducescost and increases utility.

In another aspect, the invention is a water soluble tablet compositionthat is non-hygroscopic and generates chlorine dioxide with a weight %yield of at least 20 wt % and a chlorite conversion of at least 70 wt %.

In yet another aspect, the invention allows for the tablets to be usedin a multi-tablet dispenser for on-site generation of antimicrobialsolution.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exemplary embodiment of reactor in accordance with theinvention.

FIG. 2 is the reactor of FIG. 1 after the solvent interface has beenexposed to the main solvent.

FIG. 3 is the reactor of FIG. 1 after the reactant concentrations insidethe activated reaction chambers have reached the depletion level.

FIG. 4 shows the changes at the solvent interface for a first embodimentof the reactor made with a gelling agent.

FIG. 5 shows the changes at the solvent interface for a secondembodiment of the reactor made with a binder.

FIG. 6 exemplifies bulk packaging comprising a package such as a pailcontaining a plurality of tablets comprising the composition.

DETAILED DESCRIPTION OF THE EMBODIMENT(S)

The invention is particularly applicable to generation and release ofchlorine dioxide having bleaching, biocidal, or virucidal properties andit is in this context that the invention will be described. It will beappreciated, however, that the reactor, the method of making thereactor, and the method of using the reactor in accordance with theinvention has greater utility and may be used for any other targetproduct(s). Although the main solvent is described as water for clarityof illustration, the invention is not so limited.

As used herein “enhanced weight percent yield” defines a tabletcomposition that generates chlorine dioxide equivalent to at least 20 wt% of the total tablet weight when immersed in water. The enhanced weightpercent yield can be achieved even in very dilute solutions comprisingless than 0.02 wt % of tablet composition (e.g. a 1.5 gram tablet to 8liters of water).

As used herein “food processing applications” include those aspectswithin the process that utilize antimicrobial treatments to reduce thepotential of spread of infectious disease. Applications include:vegetable and fruit washing; cleaning and sanitizing of food processingequipment; cleaning and sanitizing of animal carcasses, poultry, meat,rabbit, and egg products, treatment of poultry and animal habitats.

As used herein, “carboxylic acid donor” describes dicarboxylic acid andtricarboxylic acid that have a molecular weight between 90 and 300 gramsper mole. Examples include succinic acid, malonic acid, maleic acid,malic acid, tartaric acid, fumaric acid, glutaric acid, and citric acid.Of these, fumaric acid exemplifies a preferred polycarboxylic acid dueto its non-hygroscopic properties.

As used herein, “acid source” describes compounds that contributehydrogen ions (H⁺) when dissolved in water. Examples of inorganic acidsources include but are not limited to sodium bisulfate, potassiumbisulfate, sodium pyrosulfate, and potassium pyrosulfate. Organic basedacid sources include but are not limited to fumaric acid, succinic acidand citric acid.

As used herein, “non-hygroscopic” describes the tablet compositioncomprising the free halogen donor, acid source and chlorite donor thatresist adsorption or absorption of moisture when exposed to atmospherichumidity thereby substantially reducing the potential for the generationof chlorine dioxide. The non-hygroscopic property of the tabletcomposition can be achieved by: coating the hygroscopic components ofthe tablet composition with a film forming material exemplified bypolyvinyl alcohol; coating the said hygroscopic components with anon-hygroscopic material exemplified by magnesium carbonate light orfumed silica; and/or coating the hygroscopic components with anon-hygroscopic components exemplified by coating the sodium chloritewith non-hygroscopic fumaric acid which is used in the generation ofchlorine dioxide.

As used herein, “effective amount of combustion suppressing boron donor”defines an effective amount of boron containing compound exemplified byborax and boric acid that can reduce the combustion rate of the solidcomposition to a packing group having lower transportation and/orstorage restrictions. Division 5.1 Oxidizer Testing in accordance withthe Code of Federal Regulations, Title 49, and the United NationsTransportation of Dangerous Goods-Manual of Test and Criteria, Fourthrevised edition (2003). Solid Division 5.1 materials are assignedpacking groups using the following criteria [49 CFR .sctn.173.127(b)]:(i) Packing Group I is the sub-classification of any material which, inthe 4:1 or 1:1 sample to cellulose ratio (by mass) tested exhibits amean burning time less than the mean burning time of a 3:2 mixture, bymass, of potassium bromate and cellulose. (ii) Packing Group II is thesub-classification of any material which, in the 4:1 or 1:1 sample tocellulose ratio (by mass) tested exhibits a mean burning time less thanthe mean burning time of a 2:3 mixture, by mass, of potassium bromateand cellulose, and the criteria for Packing Group I are not met. (iii)Packing Group III is the sub-classification of any material which, inthe 4:1 or 1:1 sample to cellulose ratio (by mass) tested exhibits amean burning time less than the mean burning time of a 3:7 mixture, bymass, of potassium bromate and cellulose, and the criteria for PackingGroups I and II are not met.

The addition of an “effective amount of combustion suppressing borondonor” to the solid composition reduces the combustion rate of the solidcomposition resulting in a reducing the transportation and storagerestrictions.

“Reaction chamber” is a space that is defined by the outline of acolloidal gel wall, and includes the enclosed by the colloidal gel, thecolloidal gel itself, and any pores or channels in the colloidal gel. A“main solvent,” is any solvent that dissolves the reactant(s) andtriggers a chemical reaction. A “polymer,” as used herein, includes acopolymer. A substance that is transported at a “controlled rate” doesnot cross a physical boundary explosively all at once but gradually,over a desired period of time.

As used herein, “depletion level” indicates a predeterminedconcentration level of the reactant(s) and the target product in areaction chamber. When a reaction chamber is contacted by the mainsolvent, a chemical reaction is triggered and the reactant(s) in thereaction chamber are converted to the desired target product. The targetproduct then leaves the reactor chamber at a controlled rate. Thedepletion level may be defined by parameters other than reactantconcentration that also indicate the rate of target product generation,such as the pH level or the concentrations of the target product or abyproduct.

When the reactor wall “disintegrates,” it could collapse due to apressure difference between the inside and the outside of the reactor,dissolve in the main solvent, or come apart and dissipate due to forcesapplied by the movement in the main solvent. A membrane is a porousmaterial that allows permeation of the solvent and diffusion of theproduct. “Water,” as used herein, is not limited to pure water but canbe an aqueous solution. A gelatinous structure “dissipates” bydissolving or dispersing in the main solvent.

“Gel,” “hydrogel,” and its various derivations (i.e. gelatinous)describes a material or combination of materials that undergo a highdegree of cross-linking or association when hydrated and dispersed inthe dispersing medium (e.g. aqueous solution), or when dissolved in thedispersing medium. This cross-linking or association of the dispersedphase will alter the viscosity of the dispersing medium to a level whichrestricts the movement of the dispersing medium. As used herein,“suspension” refers to a two-phase system consisting of a finely dividedsolid dispersed (suspended) in a liquid (the dispersing medium). Gelscontain suspended particles but are different from suspensions in thatthese suspended particles create a three-dimensional structure ofinterlacing particles or solvated macromolecules that restrict themovement of the dispersing medium.

A “gel-forming material” is comprised of at least one gel-formingpolymer that, upon contact with an aqueous solution, produces a gel,hydrocolloid or hydrogel. The polymer can be natural, such as a gum(e.g. Xanthun gum), semisynthetic such as a polysaccharide (e.g.cellulose derivative), or synthetic such as a poloxamer (blockco-polymer of polyoxyethylene and polyoxypropylene), carbomer(crosslinked polymer of acrylic acid), poly (ethylene oxide) andpolyvinyl alcohol. The gel-forming material may also include otheradditives that increase the viscosity or the gel. One example is borateexemplified by disodium octaborate.

A “stiffening agent” can be water-soluble or substantiallywater-insoluble. When combined with a gel-forming material, thestiffening agent substantially reduces the dissolution rate of thetablet composition. Stiffening agents can act as a cross-linking agent.An example of a stiffening agent is polyethylene wax exemplified byLuwax sold by BASF. Borax or boric acid that is converted to boratein-situ will increase the viscosity of polyvinyl alcohol and slow thedissolution rate of the tablet composition.

A “stiffening agent comprising boron” can be any source of boroncontaining compound that cross-links with polyvinyl alcohol under theconditions achieved within the gelatinous structure. Examples includebut are not limited to: boric acid, borate in its varying hydrated formsand mixtures. Boric acid that reacts with hydroxide alkalinity releasedfrom the commercial sodium chlorite and released from the generation ofchlorine dioxide converts boric acid to borate which then cross-linkswith the polyvinyl alcohol.

A “gelatinous structure” comprises the three-dimensional hydrocolloid orhydrogel produced by the hydrolysis of the gelling agent, which mayinclude at least one natural, semi-synthetic, and synthetic polymer, aswell as any reactants or products restrained or trapped by thethree-dimensional hydrogel. The gelatinous structure describes a regiondefined by the coalesced gelatinous composition which forms the3-dimensional structure. The regional boundaries are generally definedby the innermost portion of the gelatinous composition (approaching thesurface of the remaining solid tablet) to the outermost boundary of thegelatinous composition (interfacing with the bulk of the dispersingmedium). The gelatinous structure may have a viscosity gradient acrossthe region.

A “gelling agent” defines the components required to produce thegelatinous structure. The gelling agent includes at least thegel-forming material. However the gelling agent may also include astiffening agent, pH buffer, etc.

As used herein, the term “controlled release” refers to the solidcomposition in the form of a tablet, having been contacted with water,produces and releases chlorine dioxide over an extended period of timeas opposed to a rapid release, and where the extended period of time canbe measured in seconds, minutes, hours or days depending on the size ofthe tablet and amount of gel-forming material &/or gelling agentincluded in the tablet. A tablet with equivalent amounts of reactantsunder identical test conditions without gel-forming material willdissipate in the water in less time than the tablet that includes thegel-forming material. The chemistry of the gelling agent used in thecomposition can be adjusted to control the time required to generate andrelease the chlorine dioxide.

As used herein, the term “tablet” refers to any geometric shape or sizethat comprises the components necessary to produce a solution consistingof at least chlorine dioxide, and wherein the components are gatheredtogether to form a single mass.

As used herein, the term “halogenated cyanuric acid” refers to anycombination of chlorinated or chlorinated and brominated cyanuric acidcompounds. Examples include but are not limited totrichloroisocyanurate, bromochloroisocyanurate.

As used herein, the term “slow dissolving” refers to the tablet of theinvention having a restricted rate of dissolution compared to the rateof dissolution achieved from a tablet of similar composition that doesnot comprise a gelling agent. The gelling agent restricts thedissolution of the reactants thereby slowing the rate at which thetablet dissolves, and allows for a sustained release of in-situgenerated products rather than a rapid release obtained by fastdissolving masses and powders.

As used herein, the term “free halogen donor” describes a source of freehalogen that when dissolved in an aqueous solution contributes at leastone of Cl₂, HOCl, OCl⁻, Br₂, HOBr, OBr⁻ the species of which isdependent on the solution pH and the source of free halogen donor.Example sources of free halogen donors include but are not limited tochlorinated cyanuric acid, chlorinated and brominated cyanuric acid, andbrominated and/or chlorinated hydantoin. Examples include but are notlimited to: trichloroisocyanurate, dichloroisocyanurate, potassiumchlorobromoisocyanurate, dibromodimethylhydantoin,bromochlorodimethylhydantoin, dichlorodimethylhydantoin.

As used herein, the term “free halogen” refers to free chlorinecomprising any combination or proportion of chlorine gas, hypochlorousacid and hypochlorite ions and/or free bromine comprising anycombination of bromine gas, hypobromous acid and hypobromite ions.

As used herein, the term “low solubility free halogen donor” refers to afree halogen donor having a solubility of no greater than 5 grams per100 ml of water at 25° C. Examples include but may not be limited totrichloroisocyanuric acid, dichlorohydantoin, dibromodimethylhydantoin,bromochlorodimethylhydantoin, and the like.

As used herein, the term “multi-tablet chemical dispenser” describes anyconvenient feed system that holds multiple tablets of the invention andcontacts at least some portion of the tablets with an aqueous solutionto produce a solution consisting of at least chlorine dioxide. Examplesinclude flow-thru brominators such as those sold by Great Lakes WaterTreatment, Nalco Chemical, and BetzDearborn Inc. whose disperser isexemplified in U.S. Pat. No. 5,620,671, spray feeders like those sold byArch Chemical and sold under the trade name Pulsar, floating dispensers,or a perforated dispenser such as a minnow bucket or strainer that isimmersed into the aqueous solution.

As used herein, the term “enhanced environmental stability” is definedby the solid composition's ability to substantially resist thegeneration and release of chlorine dioxide until such time that it isexposed to an aqueous solution. A solid composition with enhancedenvironmental stability substantially reduces the potential ofgeneration and release of chlorine dioxide when exposed to relativehumidity such as that experienced during production, packaging, storageand handling.

As used herein, the term “bulk packaging” defines the ability to packagea plurality of tablets into one package without segregating each tablet.Example packaging includes but is not limited to plastic bags and/orplastic pails. Bulk packaging requires the tablet possess sufficientenvironmental and chemical stability as to substantially eliminate thepotential for formation of chlorine dioxide during packaging, storageand transport.

As used herein, the term “coated” refers to the application of thegel-forming material or gelling agent onto the surface of a reactantsuch as the chlorite donor. Coated also includes encapsulation of thereactant by the gel-forming material or gelling agent by application tothe surface of the reactant using a means of spray coating, exemplifiedby, but not limited to the Wurster process of spray coating. Anothermethod of encapsulating the chlorite or other reactants using a drymethod of application of the gel-forming material is a process calledMagnetic Assisted Impact Coating (MAIC), or by simply applying a coatingin the form of a powder of gel-forming material that forms a gelmembrane when contacted with an aqueous solution. The gel-formingmaterial may also be applied onto the surface of the sodium chloritethen dried. A rotary drum drier fitted with a spraying device issuitable, however any number of spraying drying techniques can beemployed.

As used herein, the term “surrounds” refers to the free halogen donor'sposition in relation to the in-situ generating portion of thecomposition. Surrounds includes encapsulates, sandwiches as in the caseof two layers of free halogen donor with the in-situ generating portionbetween the two free halogen layers.

As used herein, the term “chlorite donor” is a substance thatcontributes chlorite anions having the formula ClO₂ ⁻ when dissolved inan aqueous solution. The chlorite donor will generate chlorine dioxidewhen reacted with hypochlorous acid and/or hypobromous acid. Example ofsuitable chlorite donors include but are not limited to is sodiumchlorite, magnesium chlorite, calcium chlorite as well as other alkalimetals chlorite salts.

As used herein, the phrase “chlorite conversion to chlorine dioxide”describes the amount of chlorite anion having the general formula ClO₂ ⁻into the in-situ generated product chlorine dioxide having the generalformula ClO₂. The amount of conversion is reported in weight percent (wt%) and is determined by dividing the amount (weight) of chlorine dioxideproduced by the total amount of chlorite anion (weight) provided by thecomposition. The equation is represented by ClO₂/ClO₂ ⁻×100=weight %

As used herein, the term “recirculating systems” describes any openaqueous system that consist of a reservoir of water and a system ofpiping to transport the water, and wherein the water transported throughthe piping is eventually returned to the reservoir. Examples ofrecirculating systems include but are not limited to: cooling systemssuch as cooling towers and cooling ponds, swimming pools, fountains andfeature pools.

As used herein, the term “biocidal solution” describes an aqueoussolution consisting of at least chlorine dioxide and results fromcontacting an aqueous solution with the slow dissolving tabletcomposition of the invention.

As used herein, the term “oxidation resistant polymer” describes apolymer possessing steric hindrance and bond strength resulting inincreased resistance to oxidation from the oxidizers in the tabletcomposition. An oxidation resistant polymer is capable of being combinedwith sodium chlorite at the ratios used in the tablet composition andtested using thermogravimetric analyses (TGA) to a temperature of atleast up to 100° C. without inducing a reaction between the sodiumchlorite and said polymer. One example of an oxidation resistant polymerincludes but is not limited to polyvinyl alcohol.

As used herein, the term “self-limiting” tablet composition describesthe tablet composition's ability to slow or stop the generation ofchlorine dioxide as the concentration of the tablet components andchlorine dioxide in the biocidal solution gets too high. Without beingheld to a particular theory, it is believed the increasing viscosityelevates the concentration of the reactants to where they reach theirsaturation level and the tablet slows its dissolution rate.

As used herein, the term “water” includes aqueous solution(s) thatcomprise water, but is not limited to strictly water having the generalformula H₂O, wherein “H” is Hydrogen and “O” is Oxygen. The use of theterm “water” is not meant to imply limitations to the use of thedisclosed solid composition with respect to the quality of the water inan aqueous solution. An aqueous solution may contain contaminants,minerals, dissolved and suspended solids.

As used herein, the term “carboxylic acid” describes a an acid sourcehaving at least two carboxylic acid functional groups having the generalformula COOH, wherein “C” is carbon, “O” is oxygen, and “H” is hydrogen.

The invention is based on the discovery that a solid tablet compositioncan be produced to provide an enhanced weight percent (wt %) yield ofchlorine dioxide. The enhanced weight percent yield is obtained bycombining select reactants that provide an optimum chemistry forgenerating chlorine dioxide with a gel-forming material comprising atleast one of a natural, semi-synthetic and synthetic polymer thatencapsulates the high solubility chlorite donor, and entraps thereactants within a gel matrix that functions as a membrane.

When the selective reactants are first exposed to a small volume ofwater and allowed to react to generate the target product, a high yieldof the target product can be obtained because the reactantconcentrations are high and the chemistry supports efficient conversionof chlorite anions to chlorine dioxide. Then, the chlorine dioxide canbe exposed to a larger volume of water. The rate at which the reactantsare exposed to water has to be such that the chlorine dioxide isgenerated in high yield before more water dilutes the reactants. Theinvention controls the dissolution of the reactants' upon exposure towater by forming a gel layer that functions as a temporary coating thatkeeps the reactants in intimate contact for a sufficient time period toallow the reactions to proceed toward completion. It is believed bycoating at least the high solubility chlorite donor, the chlorite anionsresulting from dissolution in water are restricted from dissipating awayfrom the tablet and other reactants. The coating of the high solubilitychlorite allows for addition of high amounts of chlorite donor into thesolid composition which then effectively induces formation of a gelmatrix across the wetted surfaces of the tablet thereby entrapping otherreactants. Additional gel-forming material can be added to the solidcomposition which can be especially useful when using lower amounts ofcoated chlorite donor and/or using other high solubility reactants thatwould otherwise quickly dissipate and disperse.

Thus far, the oxidizing power of chlorine dioxide has not been fullyexploited because the cost of equipment to produce chlorine dioxidein-situ to the application is prohibitively high. Also, when usingconventional powders or tablets, the economics are severely compromiseddue to poor “weight % yield” of the powders and tablets as well as thecost of producing these chlorine dioxide generators. The poor “weight %yield” is demonstrated in the '404 Patent discussed above. Furthermore,the utility of chlorine dioxide tablets is compromised due to the poorenvironmental stability which results in individually wrapped tablets.

The ability to produce a tablet composition that: generates a highweight % yield of chlorine dioxide; has substantially improvedenvironmental stability so that it can be packed in bulk whereinmultiple tablets can be combined into one package rather thanindividually wrapped; have a controlled release rate when immersed inwater to provide chlorine dioxide over an extended period of time; andbe self-limiting so that the dissolution rate of the tablet compositionsubstantially slows or stops as the concentration of the tabletcomposition components in the biocidal solution is substantiallyelevated, provides a tablet composition that eliminates the existingbarriers for use of chlorine dioxide in multi-tablet dispensers.

Chlorine Dioxide

In one embodiment, the composition comprises: a chlorite donorexemplified by sodium chlorite having from approximately 34-69 wt % ascommercial sodium chlorite based on having an 82 wt % sodium chloriteactivity (approximately 20.6-42.0 wt % as chlorite anion); a freehalogen donor exemplified by trichloroisocyanurate (TCCA) and rangingfrom 12-60 wt %, and in sufficient amount to convert at least 70 wt % ofthe chlorite anion to chlorine dioxide; an acid source comprising from3-50 wt %, and in sufficient amount to provide a pH of less than 7.8when 1 gram of tablet composition is dissolved in 100 ml of water; agelling agent comprising from 0.1 wt % to 30 wt % of a gel-formingmaterial comprising at least one of a natural, semi-synthetic, andsynthetic polymer.

Without intent to limit the sources and types of gel-forming material,examples include polyvinyl alcohol sold under the trade name Elvanol® byDuPont, poly (ethylene oxide) sold under the trade name Polyox® by DowChemical, Poloxamer sold under the trade name Pluronic® by BASF,Carbomer sold under the trade name Carbopol® by Noveon, polysaccharidesexemplified by Xanthan gum sold by Ingredient Solutions, Inc., and watersoluble cellulose derivatives sold by Eastman. These examples comprisenatural, semi-synthetic and synthetic polymers that increase theviscosity when contacted with an aqueous solution. The preferredgel-forming material is an oxidation resistant polymer exemplified bypolyvinyl alcohol. The gelling agent may include a cross-linking agentexemplified by borax in the case of polyvinyl alcohol, a stiffeningagent exemplified by polyethylene wax, or can be used alone. Thecomposition provides a controlled release solid composition in the formof a tablet for use in multi-tablet dispensers and provides a conversionof chlorite anions to chlorine dioxide of at least 70 wt %. The use ofthe gelling agent also allows for sufficient chlorite donor to generatea yield of chlorine dioxide of at least 20 weight % while providing asubstantially water soluble composition. A weight % yield of at least30% has been achieved from tablets of this invention.

The preferred chlorite donor is sodium chlorite. However other chloritedonors that provide chlorite anions (ClO₂ ⁻) when dissolved in watercould be used in the composition.

Free halogen donors contribute halogen based oxidizers when contactedwith an aqueous solution. For example, Trichloroisocyanuric acid (TCCA)releases free chlorine as it is dissolved by water. The species of thefree chlorine is dependent on the pH of the solution. The species offree chlorine can include Cl₂, HOCl, and OCl⁻. The species of freebromine can include Br₂, HOBr, and OBr⁻.

An acid donor consumes the hydroxide alkalinity released from theformation of chlorine dioxide and released from the chlorite donor. ThepH of the resulting biocidal solution was illustrated in the exampletest of the referenced co-pending applications. Acid sources can beorganic and inorganic. Examples of organic acid sources include but arenot limited to cyanuric acid, succinic acid, fumaric acid, tartaricacid, and citric acid. The preferred acid source is a dicarboxylic ortricarboxylic acid exemplified by fumaric acid and citric acidrespectively. Fumaric acid is an example of a preferred organic acidhaving limited solubility and being substantially non-hygroscopic.Examples of inorganic acid sources include but are not limited to sodiumbisulfate potassium bisulfate, sodium pyrosulfate and the like.

The gelling agent will be comprised of at least a polymer that, uponcontact with an aqueous solution is capable of producing a hydrogel. Thepolymer can be natural, such as a gum (i.e. Xanthun gum), semisyntheticsuch as a polysaccharide (i.e. cellulose derivative), or synthetic suchas a poloxamer, carbomer, poly (ethylene oxide), polyvinyl alcohol andthe like. The preferred polymer is an oxidation resistant polymer thatis exemplified by polyvinyl alcohol and carbomer. Oxidation resistantpolymers such as polymers with cross-linking exemplified by polyvinylalcohol and Carbomers reduce the potential for reacting with thereactants or the biocidal solution produced. Additional stiffeningagents such as incorporating borax with polyvinyl alcohol can be used toincrease the viscosity and further reduce dissolution rates of thereactants.

Additional additives such as pH buffers maybe included depending on thereactants used as well as the final application. For example in caseswhere trichloroisocyanuric acid (TCCA) is included, the acidity from theTCCA may be sufficient to neutralize the alkalinity from the chloritedonor. However, in applications where dibromodimethyl hydantoin (DBDMH)is used, additional acidity maybe required since DBDMH has a pH nearneutral.

Optional Components in the Reactor

1) Fillers

Fillers can be used or altogether omitted depending on the type ofprocessing and the requirements of the use of the final product. Fillersmaybe inorganic compounds such as various mineral salts, metal oxides,zeolites, clays, aluminates, aluminum sulfate, polyaluminum chloride,polyacrylamide, and the like.

2) pH Buffers

A pH buffer provides a source of pH control within the reactor. The pHbuffers can be inorganic (e.g. sodium bisulfate, sodium pyrosulfate,mono-, di-, tri-sodium phosphate, polyphosphates, sodium bicarbonate,sodium carbonate, boric acid, borax, and the like). Organic buffers aregenerally organic acids with 1-10 carbons such as succinic acid.

The pH buffer can be employed to adjust the pH of the solution resultingfrom the dissolution and reactions resulting from the composition inorder to achieve the desired conversion of chlorite to chlorine dioxideand/or achieve the desired viscosity of the gel.

3) Cross-Linking Agents

Cross-linking agents, which are additives that change the physical orchemical properties of the composition, may be added to the reactor tocontrol (e.g., reduce) the dissolution rate of the composition. Forexample, glycoluril is effective at bonding with hydroxyl and carboxylicacid groups such as those found in the cellulose of hydrolysedsilicates. Glycerin and borates alter the water permeation rate ofpolyvinyl alcohol. Therefore, these types of agents can be added or leftout depending on the final dissolution rate, hygroscopicity, chemicalresistance to oxidizers, etc.

A cross-linking agent is mixed with the binder, and the mixture iscombined with the reactants in the manner described above. In caseswhere curing is required to set the cross-linking agent, the binder andthe cross-linking agent are combined in the presence of a solvent and/ora curing agent, mixed, and reacted. If needed the mixture is dried priorto application (e.g., being combined with the reactants).

4) Stiffening Agent

The stiffening agent is used to increase the viscosity of the gel andfurther slow the dissolution rate of the tablet. Polyethylene wax suchas Luwax® sold by BASF is an example of a stiffening agent that furtherslows the dissolution of the tablet. Many hydrocarbon based polymerswith low solubility or considered insoluble in water will serve as astiffening agent. Cross-linking agents can also function to stiffen thegel as a result of increasing the gel viscosity and thereby reducing thedissolution rate of the tablet.

5) Surfactants

In some instances surfactants can be incorporated into the compositionto reduce the dissolution rates of the higher solubility reactants aswell as provide a synergistic effect in combination with the biocidalsolution. For example, a block copolymer surfactant exemplified byPluronic® manufactured by BASF can reduce the dissolution rate of thereactants as well as provide surfactant to the biocidal solution toenhance the performance of the chlorine dioxide by increasing thepenetration of biofilms and membranes of microbiological organisms.Other examples include poly (ethylene oxide) sold by Dow Chemical underthe name Polyox.

6) Other Additives

Other additive such as lubricants and potentially binders can be addedto enhance the manufacturing of the tablet.

7) Anti-Caking Agents

It may be advantageous to apply an anti-caking agent exemplified bymagnesium carbonate light, untreated fumed silica and treated fumedsilica. Fumed silica is sold under the trade name CAB-O-SIL® and ismanufactured by Cabot Corporation. Anti-caking agents can also reducethe hygroscopic nature of sodium chlorite as well as the entire solidcomposition.

A. Structure

FIG. 1 is an exemplary embodiment of the chlorite donor coated with agel-forming material that encapsulates the chlorite donor.

FIG. 2 is an exemplary embodiment of reactor 10 in accordance with anembodiment of the invention. Although the reactor 10 in this exemplaryembodiment is cylindrically shaped, the invention is not so limited. Thereactor 10 is an aggregate composition containing one or more reactants12 and a chlorite donor encapsulated with a gel-forming material 14.Although the reactants 12 and the chlorite donor encapsulated with agel-forming material are shown only for a solvent interface 16 of thereactor 10, they are preferably present throughout the reactor 10. Thegel-forming material of 14 forms a gel when it comes in contact with themain solvent. Thus, when the reactor 10 is placed in contact with themain solvent, the gelling agent in the parts of the reactor 10 that comein contact with the main solvent will form a viscous gel that entrapsthe reactants 12, and divide the wet parts of the reactor 10 intomultiple reactor chambers. The gel functions as a membrane allowing somepermeation of the main solvent and other fluids across it, but in arestricted manner. Although only one interface 16 is shown in thisexample for simplicity of illustration, there may be multiple interfacesbetween the reactor 10 and the main solvent; in fact, the reactor 10 maybe placed in a bulk body of main solvent resulting in its completeimmersion.

FIG. 3 is an exemplary embodiment of reactor 10 in accordance with anembodiment of the invention. Although the reactor 10 in this exemplaryembodiment is cylindrically shaped, the invention is not so limited. Thereactor 10 is an aggregate composition containing one or more reactants12, and gelling agent 13, and a chlorite donor encapsulated with agel-forming material 14. Although the reactants 12, gelling agent 13,and the chlorite donor encapsulated with a gel-forming material 14 areshown only for a solvent interface 16 of the reactor 10, they arepreferably present throughout the reactor 10. The gel-forming materialof 14 and gelling agent forms a gel when it comes in contact with themain solvent. Thus, when the reactor 10 is placed in contact with themain solvent, the gelling agent in the parts of the reactor 10 that comein contact with the main solvent will form viscous gel that entraps thereactants 12, and divide the wet parts of the reactor 10 into multiplereactor chambers. The gel functions as a membrane allowing somepermeation of the main solvent and other fluids across it, but in arestricted manner. Although only one interface 16 is shown in thisexample for simplicity of illustration, there may be multiple interfacesbetween the reactor 10 and the main solvent; in fact, the reactor 10 maybe placed in a bulk body of main solvent thereby resulting in completeimmersion.

FIG. 4 is the reactor 10 after the solvent interface 16 has been exposedto the main solvent. As shown, gel 18 is formed at the interface betweenthe main solvent and the reactor 10. The gel 18, which forms reactionchambers at the interface 16, restricts the diffusion of fluids acrossit. Thus, the environment inside of the reaction chambers is differentfrom the bulk main solvent body outside the reactor 10. The environmentinside the reactor 10 is more conducive to efficient chlorine dioxidegeneration than the bulk main solvent environment. While the gel wallsform in parts of the reactor 10 that is in contact with the mainsolvent, the dry parts of the reactor 10 remain substantially in theiroriginal form.

FIG. 5 is the reactor 10 after the reactant concentrations inside theactivated reaction chambers have reached the depletion level. When thedepletion level is reached, the gel walls 18 that form the reactionchambers begin to dissipate or disintegrate, as shown with dotted linesto indicate the dissipation of the colloidal gel. Since the gel wallsrestricts the main solvent from contacting the deeper portions of thereactor 10, remain substantially dry while the first layer of gelreaction chambers are generating the chlorine dioxide. The dissipationof the gel walls 18, however, causes the layer of reactants andgel-forming material and/or gelling agent that was under the gel layerto come in contact with the main solvent. This newly exposed part of thereactor 10 then contacts the main solvent, forms another set of gelwalls, generates the chlorine dioxide, and releases the chlorinedioxide. The next level of reactants-and-gelling agent then comes incontact with the main solvent, and the generation and release of thechlorine dioxide continues as layers of the reactor 10 are dissipatedinto the main solvent body.

The invention includes a method of preparing the reactor. The reactorproduces high concentrations of chlorine dioxide that is different fromthe reactants that are initially present in the reactor. The method ofthe invention allows the production of compositions that are stable forstorage and, upon activation by contact with water, produce chlorinedioxide in an elevated weight percent yield.

One method for preparing the reactor 10 entails: encapsulating thechlorite donor with a gel-forming material; mixing the encapsulatedchlorite donor with reactants and optional gelling agents, fillers,lubricant and the like; and feeding the mixture to agglomeratingequipment. Once fed to the agglomerating equipment, a force is applied.The pressure results in the formation of a solid tablet composition. Theexact force to be applied is determined based on the final composition,the desired density of the resulting agglomerate, the desireddissolution rates, and the like.

Examples of equipment suitable for producing the solid composition inthe form of a tablet may include but may not be limited to: rollcompactors; tablet presses; agglomerator; briquetting machines and thelike. Suitable equipment is obtainable thru a large number of companiesexemplified by GEA Process Engineering Inc, Columbia, Md. 21045, and SMIIncorporated, Lebanon, N.J. 08833.

FIG. 6 is exemplary of bulk packaging but is not intended to limit thetypes of packaging, multiple layers of packaging that can be implementedto reduce damage to the solid tablet composition, or solid tabletconfiguration. Additional layers of packaging may include but notlimited to: plastic liners; inclusion of desiccant packets such assilica based desiccant; and gas purging of the package such as nitrogengas purging. Illustration 20 represents a container or packaging intowhich the solid tablets 21 are contained. A lid or enclosure, morepreferably a sealable lid or enclosure is used to reduce the potentialfor exposure to the external environment.

Gelling Agent

Gelling agents are combined with the reactants to form a mixture.Gelling agents, upon exposure to the main solvent, form a gel that ispermeable to the main solvent. Examples of gelling agents include butare not limited to: polysaccharides including cellulose; water absorbentpolyacrylic polymers and copolymers such as Carbopol® sold by Noveon,Inc.; poloxamer block copolymer such as Poloxamer 407 sold by BASF underthe trade name Pluronic®; polyvinyl alcohol sold under the trade name“Elvanol” by DuPont; poly(ethylene oxide) such as Polyox™ sold by DowChemical, can be used. Gelling agents can be a stand-alone polymer asillustrated or a combination of components that may include a stiffeningagent or cross-linking agent that slow the dissolution rate of thetablet and/or increase viscosity.

Gelling agents comprise at least one gel-forming polymer. Thegel-forming polymer can be natural, such as a gum (e.g. Xanthun gum),semisynthetic such as a polysaccharide (e.g. cellulose derivative), orsynthetic such as a poloxamer (block co-polymer of polyoxyethylene andpolyoxypropylene), carbomer (crosslinked polymer of acrylic acid), poly(ethylene oxide) and polyvinyl alcohol. The preferred gel-formingpolymers are synthetic, as synthetic polymers exemplified by polyvinylalcohol and ethylene modified copolymer sold under the trade nameExceval® and manufactured by Kuraray Specialties Europe demonstrate highlevels of oxidation resistance. However, natural and semisyntheticpolymers may be used especially as an additional gel-forming polymer, orin cases where the oxidizers are precoated with a barrier film, such asfumed silica, magnesium carbonate light, borates, and the like.

B. Gelatinous Structure

A tablet may be prepared with a gel-forming material that forms agelatinous structure when exposed to the main solvent. When thegelatinous structure is formed, so do chambers as described above. Thechambers contain reactants that produce the target product from anin-situ chemical reaction, allowing for substantially higher yields ofproduct than tablets having equivalent mass with no gel-formingadditive. The gelatinous structure may disintegrate and dissipate afterthe chamber contents are depleted. The gelatinous structure functions asa membrane.

Using the gelatinous structure of the invention, water-solublecompositions for generating chlorine dioxide can be produced that yieldconcentrations of chlorine dioxide over 250% more than the existingwater-soluble compositions. Further still, the disclosed inventionincreases the weight percent yield of chlorine dioxide to at least 20 wt% as illustrated in the data section of this disclosure and can achievein excess of 30 weight percent yield of chlorine dioxide. A tabletcomposition capable of generation a weight percent yield of at least 20wt % of chlorine dioxide is unprecedented. Furthermore, the gel-formingmaterials allow for a tablet composition with substantially improvedenvironmental stability. These characteristics can allow for packagingin bulk by combining multiple tablets in one package. Further still, theuse of a gel-forming material provides for a delayed effect thatrequires a period of elapsed time before any reaction occurs. Thischaracteristic also allows for the controlled release of the chlorinedioxide allowing for use in a multi-tablet dispenser. Water-soluble gelsprovide superior weight percent yield and improved utility over theexisting compositions that utilize high concentrations of inertmaterials (e.g., swelling clays) to construct a porous structure.

Gel forming additive technology can be readily assimilated into otherin-situ generating tablets to achieve the same benefits in yield byincreasing the weight % of reactants in the agglomerate composition, andresulting in an increase in the weight % yield of desired product.

The invention is based on the use of gel-forming material that bind andhold the reactants together while immersed in the main solvent, therebymaintaining the structural integrity of the agglomerate composition. Thegel, which is formed when the gel-forming material contacts the mainsolvent, restricts the diffusion of the reactants until the reaction isnear complete. After the reaction is substantially complete, the geldisintegrates.

The gel-forming materials are particularly useful in producingself-sustaining tablets that produce high yields of in-situ generatedoxidants. The use of this gel-forming technology dramatically reducesthe quantity of inert materials used to improve reaction kinetics inprior art, and substantially increases the “weight % yield” of thetablet when compared to tablets incorporating currently known methods.

Without limiting the invention, useful components used in formingwater-soluble gels include: natural, semi-synthetic and syntheticpolymers such as Polyvinyl alcohol (PVA) or cross-linked polyacrylatessold under the trade name Carbopol® by Novean, and copolymers such aspolyoxyethylene polyoxypropylene block copolymer sold under the tradename Pluronic® by BASF, poly (ethylene oxide) exemplified by Polyox soldby Dow chemical, polysaccharides, gums, and water soluble cross-linkingagents such as borax in the case of PVA.

Composite gels are particularly useful in that small quantities relativeto the total mass of the agglomerate composition dramatically improvethe structural integrity of the agglomerate when immersed in water, andimprove the weight % yield of the agglomerate composition. Compositegels contain at least two additives that, when combined, produce agelatinous structure having a viscosity substantially higher than thatobtained using either the additive alone when exposed to the sametemperature an pH conditions. Composite gels are produced by combining aviscosity-increasing material with an additive that enhances theformation and rigidity of the gel. For example, a composite gel may be acombination of PVA and borax. When composite gelling agents are used,the number and types of compounds that can be used increases. Also, theamount of viscosity modifying agent can often be substantiallydecreased.

For example, while PVA increases the viscosity of the solution, it haslimited effect on the dissolving rate of the tablet at normal useconcentrations. Elvanol® sold by DuPont typically shows a viscosityprofile of up to approximately 2,000 centipoise at a 10 wt. % solution.However, combining borax or boric acid with an alkali and the PVAproduces a gelatinous composite having a viscosity over 100,000centipoise. Composites can also be produced by combining multiplegel-forming materials to produce a gel of substantially higher viscositythan when the compounds are used with pH buffers as illustrated in theexamples.

Gelatinous Structure

In the gelatinous structure capable of producing in-situ generatedoxidants in high yield, the viscosity of the gel is sufficiently high toprevent diffusion of the reactants and gel even under conditions thatwould normally induce rapid dilution. The viscosity being sufficientlyhigh also helps maintain the structural integrity of the agglomerate asto prevent a premature breakup of the agglomerate composition whenimmersed in excess diluting solvent. The gel rigidity is preferably at alevel that is sufficient to prevent rapid dispersion of the agglomerateeven when agitation or circulation of the water occurs. To achieve this,it is desirable to utilize a gel-forming chemistry that produces a“gelatinous structure” within the agglomerate. This gelatinous structurehas a viscosity greater than about 500 centipose, preferably greaterthan about 5,000 centipoise, and preferably greater than about 50,000centipoise. The level of viscosity can be altered to achieve the desireddissolving rate of the tablet.

The gelling agent constitutes no more than 30 wt % of the solidcomposition, preferably constitutes less than about 10 wt % of the solidcomposition.

Advantages to Combining Gel-Forming Agents

High solubility reactants such as sodium chlorite and succinic acid whencombined with TCCA results in a tablet that when combined with waterforms large pores, caverns, and tends to release the high solubilityreactants faster than the slower dissolving TCCA. The channels that formcan still allow good conversion of chlorite to chlorine dioxide howeverthey compromise the structural integrity of the tablet, thereby makingit brittle and crumble under the weight of other tablets in amulti-tablet dispenser. Furthermore, the mechanism that provides acontrolled-release as well as a self-limiting tablet is alsocompromised. These characteristics are undesirable for tablets that areto be used in a multi-tablet dispenser, but may be satisfactory assingle or multiple tablet applications that make a single batch ofbiocidal solution.

By coating or encapsulating the chlorite donor with a gel-forming agentexemplified by polyvinyl alcohol, the environmental stability is greatlyenhanced and it reduces the potential of reaction between the chloritedonor and other reactants during manufacturing and storage. By applyinga coating onto the sodium chlorite comprising a mixture of partiallyhydrolyzed polyvinyl alcohol (exemplified by Elvanol 51-03) with a fullyhydrolyzed polyvinyl alcohol (exemplified by Elvanol 71-30), results ina solid composition in the form of a tablet with improved environmentalstability and suppressed dissolution rate than when Elvanol 51-03 isused alone. By further including another gel-forming agent such as poly(ethylene oxide) exemplified by Polyox WSR N-750 into the mix ofreactants that includes the free halogen donor exemplified by TCCA, thedissolution rate of the tablet is dramatically decreased, and the tablettakes on a self-limiting characteristic that limits the maximumconcentration of dissolved reactants and in-situ generated biocidalsolution on a multi-tablet dispenser.

Applying the Gelling Agent

The gelling agent can be mixed with the other components prior toforming a tablet or agglomerate. A ribbon mixer, tumbler or anyconvenient commercially viable means of applying the coating to thereactant(s) may be used. It is preferred the polymer coating is in theform of a powder to effectively coat the chlorite donor. Polymer such asPolyvinyl alcohol (PVA) can be difficult to pulverize. Spray drying asolution of polymer exemplified by PVA results in a micronized powderthat enhances the coating of the chlorite donor and when applicabledistribution within the tablet.

In another application, the gelling agent can be applied to the surfaceof the reactant(s) having the higher solubility thereby forming acoating, followed by mixing the coated reactant(s) with the othercomponent(s) that have lower solubility. The coating may be applied bysimply mixing the gelling agent and reactant together, or by physicallyattaching the gel-forming material to the surface of the reactant byusing methods such as Magnetically Assisted Impact Coating (MAIC).

In yet another application, the gelling agent is applied to the surfaceof at least the chlorite donor by spraying a solution of the gellingagent onto a surface of the chlorite donor in a fluid bed coating systemfollowed by drying. A suitable method is exemplified by the Wursterprocess wherein the solid chlorite donor is suspended in a stream ofheated air and a solution of gelling agent is sprayed onto the surfaceof the chlorite donor where it is then dried in the stream of airthereby encapsulating the chlorite donor.

In another application, the gel-forming polymer exemplified by polyvinylalcohol can be mixed into a solution of sodium chlorite, after which themixed solution is dried resulting in a matrix of polyvinyl alcohol andsodium chlorite.

Coated Chlorite with Powdered Polyvinyl Alcohol

Commercial 82% granular sodium chlorite was coated with a mixturecomprising 89.8 wt % powder Elvanol 52-22 (polyvinyl alcohol) and 10 wt% Pluronic 127prill (poloxamer) that had been intimately mixed bycombining the components in a coffee grinder and grinding until ahomogenous mix resulted. Then 0.2 wt % of Neobor Tech Powder obtainedfrom U.S. Borax Inc. and mixed in to complete the gelling agentcomposition.

Granular sodium chlorite was dried at 50° C. for 2 hours and removedfrom the drier. While still warm, the sodium chlorite granules wherecombined with the gelling agent and extensively mixed to provide 4 wt %of coating. The coated sodium chlorite was allowed to cool and rest forapproximately 2 hours by placing into a container containing silicadesiccant and covered.

A batch of components was produced by combining 56 wt % coated sodiumchlorite (52 wt % commercial sodium chlorite), 12 wt % succinic acid, 32wt % TCCA and mixing extensively until uniform.

3 tablets each weighing approximately 3 grams were produced using aCarver tablet press using the methods previously disclosed.

Enhanced Weight Percent Yield and Environmental Stability

3—tablets were exposed to room conditions and tested in one weekincrements. None of the tablets showed any sign of gas generation duringthe exposure period. Each tablet was tested by adding to a plasticbucket 17.5 liters of tap water. One tablet was dropped into the centerof the bucket and allowed to settle. The bucket was then closed byplacing a sealable lid on top and sealing. The bucket was allowed torest undisturbed for 24 hours. After 24 hours the lid was removed, thewater was swirled to disperse the chlorine dioxide rich solution whichaccumulated in the bottom portion of the water. A sample was removed andthe chlorine dioxide concentration was determined using a HACH DR 2000spectrophotometer.

Week wt ClO2 wt % Yield % Conversion 1 3.0 51.6 ppm 30.1 95.4 2 2.9 49.0ppm 29.5 93.7 3 2.9 49.5 ppm 29.9 94.7Enhanced Stability with 4 wt % and 10 wt % PVA Coating

tablet sodium chlorite max ClO2 chlorine dioxide Sample Date (g) wt %water (L) (g) conc ppm grams wt % conversion % A  6-May 1.524 0.608 10.509 511 0.511 33.5 100 B  6-May 1.528 0.608 1 0.51 520 0.52 34 101 C 7-May 1.63 0.608 1 0.533 541 0.541 33.2 101 D  7-May 1.513 0.608 10.495 507 0.507 33.5 102 E 29-Apr 1.561 0.6 1 0.548 561 0.561 36 102 F29-Apr 1.575 0.6 1 0.553 573 0.573 36 103 G 29-Apr 1.49 0.6 1 0.524 5340.534 36 101 A/B SC (10% PVA) TCCA FCWS 0.62 0.23 0.17 C/D SC (10% PVA +2% MC) 0.62 0.23 0.17 E/F/G SC (4% PVA) 0.6 0.23 0.17Encapsulated Chlorite Using Wurster

Four Kilograms of commercially available granular sodium chloritereported as 82% sodium chlorite was sent to Aveka, Inc. along with asample of Elvanol 51-03 (polyvinyl alcohol or PVA).

Aveka, Inc first tested a mixture comprising 96 wt % sodium chlorite and4 wt % Elvanol 51-03 using thermogravimetric analyses (TGA). The testwas conducted up to 200° C. and showed that the oxidation resistantpolymer (polyvinyl alcohol) and sodium chlorite had no reaction.

Aveka used a Wurster coater (Vector FL-M-1 unit) to apply a solution ofElvanol 51-03 to the fluidized granular sodium chlorite. Two samples ofencapsulated sodium chlorite were produced. One sample had 2 wt % ofElvanol 51-03 applied while the other sample had 4 wt %. The wt % isestimated based on the amount of Elvanol 51-03 applied to the sodiumchlorite and assumes all of the Elvanol 51-03 was effectively applied tothe fluidized sodium chlorite.

10.84 grams of 4 wt % PVA coated sodium chlorite and 2.4 grams ofsuccinic acid with a particle size of <425 micron was combined andmixed. 0.27 grams of Carbopol 676 was added to the mixture andthoroughly mixed to coat both the PVA-sodium chlorite and succinic acid.1.2 grams of Polyox WSR N-750 was added and mixed. 6.4 grams ofTrichloroisocyanuric acid having a particle size of <180 micron wasadded and the composition was thoroughly mixed.

Three tablets were produced by adding 5 grams of the mixture to a 16 mmdie and pressed using a Carver Laboratory Press using 10,000 lbs offorce resulting in a tablet having a cylindrical shape.

Self-Limiting Solid Composition in Tablet Form

One tablet weighing 5.00 grams was added to 25 ml glass vial with aplastic screw on cap. Water was added until the vessel was completelyfilled. A piece of plastic wrap was applied over the top and the vesselwas sealed with the cap. The closed vessel was immersed into a twogallon pail of water for safety purposes.

The tablet was allowed to sit undisturbed for 24 hours. After 24 hoursthe vessel was held underwater and the lid was removed, and the contentscontaining a thick viscous solution of bright yellow chlorine dioxidewhere spilled out. The chunk of remaining tablet was removed, rinsed toremove loose gel, and dried with a paper towel. The sample weight was1.77 grams.

Another tablet was added to 1000 ml of water and the beaker was coveredwith plastic wrap. The tablet was completely dissolved in 11 hours 45minutes.

Delayed Reaction

Another 5 gram tablet was dropped into 3500 ml of 80° F. water. Thetablet landed on the bottom and required and additional 5 seconds beforeany indication of chlorine dioxide formation was detected. Subsequenttest utilizing a 10 wt % PVA coating of chlorite provided 10 seconds ofdelay before the generation of chlorine dioxide was observed.

Enhanced Weight Percent Yield

Three tablet compositions for each of three different water solubleformulations were produced. The chlorite was encapsulated with 4 wt %PVA. The ratios of components were as follows:

Formula 1 Formula 2 Formula 3 chlorite 0.54 g chlorite 0.6 chlorite 0.54TCCA .0.32 g TCCA 0.2 TCCA 0.30 fumaric 0.14 g fumaric 0.2 fumaric 0.16sodium chlorine chlorite dioxide tablet (g) wt % moles water (L) concppm grams moles conversion % Formula 1 5.129 54 0.02523 2 845 1.690.02522 100 5.03 54 0.0247 6 273 1.638 0.0244 100 9.658 54 0.0475 8 3953.16 0.04716 100 Formula 2 4.987 60 0.02726 3 519 1.557 0.02323 89.24.996 60 0.027311 2 839 1.678 0.02504 96.1 Formula 3 5.06 54 0.02489 3545 1.635 0.0244 98 5.07 54 0.0249 2 837 1.674 0.0249 100

The non-hygroscopic fumaric acid made a very environmentally stabletablet compared to the succinic acid. Formula 2 illustrates a tabletcomposition with excess chlorite that would deplete virtually all of thefree chlorine, thereby resulting in an antimicrobial solution with verylittle potential for generating trihalomethanes when used in foodprocessing applications. The sodium chlorite provides a minimum 82 wt %sodium chlorite. In cases where the calculated chlorine dioxide wasgreater than 100 wt %, the discrepancy may be attributed to a higheractivity of chlorite in the commercial product than that reported as theminimum on the label.

Chlorite Limits for Achieving 70% Conversion

tablet max ClO2 conc (g) wt % water (L) (g) ppm grams conversion %Formula A 1.511 61 1 0.5401 516 0.516 95.5 1.512 61 2 0.5405 510 0.5194.3 4.99 61 2 1.7837 822 1.644 92.17 Formula B 1.5 65 2 0.5713 2560.512 89.6 1.5 65 1 0.5713 505 0.505 88.39 1.506 65 1 0.5736 509 0.50988.73 1.501 65 1 0.5717 500 0.5 87.4 Formula C 4.97 70 2 2.0386 685 1.3767.2 1.497 70 1 0.614 437 0.437 71.1 Formula D 1.521 61.9 1 0.5406 5360.536 99.1 1.491 61.9 2 0.53 257 0.514 96.9 1.515 61.9 3 0.5385 1760.528 98 5.048 61.9 6 1.794 281 1.686 94 5.012 61.9 8 1.782 213 1.70495.6 2.002 61.9 0.8 0.7116 851 0.68 95.5 SC (4% pva) TCCA FCWS PolyoxFormula A 0.61 0.22 0.14 0.03 Formula B 0.65 0.2 0.13 0.02 Formula C 0.70.25 0.05 Formula D** 0.6 0.2 0.17 max max max tablet sodium chlorite SCClO2 ClO2 ClO2 (g) wt % water (L) (gm) (gm) mg/ltr ppm wt % Conversion %Formula B 1.517 72 8 1.0486 0.6363 79.53  54 28.4 67.9 1.549 72 3.51.0707 0.6497 185.63 132 29.8 71.11 1.52 72 2 1.0506 0.6375 318.75 22129.1 69.33 1.527 72 1 1.0555 0.6405 640.48 463 30.3 72.29 NaClO2 4% PVATCCA Fumaric Formula B 0.72 0.2 0.08 **used 2% MgCO3.

These sets of data show the range limits, enhances weight percent yield,and various conversions of chlorite anion to chlorine dioxide forvarious tablet formulations using Trichloroisocyanuric acid (TCCA),Sodium Chlorite (SC), and Fumaric acid, (FCWS).

The disinfection requirements of an open recirculating industrialcooling system are markedly different from those of a potable watertreatment facility. The disinfection goal of potable water facilities isthe sterilization of water as measured by specific water bornepathogens. The goal of disinfection for industrial cooling systems isthe removal or minimization of any biofilm, which retards heat transfer,causes biofouling, provides a place of agglomeration for marginallysoluble or insoluble salts, and provides a place which nurtures andpromotes the growth of highly corrosive anaerobic bacteria.

Many researchers have cited the excellent biofilm removing properties ofchlorine dioxide. In at least one previously reported case history, theintroduction of chlorine dioxide into a heavily fouled cooling systemresulted in an increase in both turbidity and calcium. These wereexplained by a dispersing of the biofilm which both increased turbidityand released small calcium carbonate particulates which had been trappedin the biofilm.

Other industries have made use of the excellent biofilm removalproperties of chlorine dioxide, particularly the food industry. Smallcooling towers, frequently contaminated by food products or by-products,have tremendous slime forming potential. Chlorine dioxide has achievedwidespread usage in such systems, due to its excellent biofilmdispersing/bacterial disinfecting properties.

The composition of the invention is effective as a biocide and algaecidetreatment for use in recirculated water systems such as industrialcooling systems and swimming pools. While the foregoing has been withreference to a particular embodiment of the invention, it will beappreciated by those skilled in the art that changes in this embodimentmay be made without departing from the principles and spirit of theinvention.

1. A solid composition in the form of a tablet that produces chlorinedioxide on demand upon contact with water, the composition comprising:at least one solid chlorite donor in an amount to provide chlorite anionto obtain at least 25 wt % yield chlorine dioxide when the compositionis contacted with water; at least one solid form of low solubility freehalogen donor in an amount from 12 to 60 wt % and in an amount toconvert at least 70 wt % of the chlorite anion to chlorine dioxide whenthe composition is contacted with water; at least one solid acid sourcein an amount to provide a pH of less than 7.8 when 1 gram of tabletcomposition is dissolved in 100 ml of water; and a coating covering thechlorite donor, the coating comprising at least one gel-forming polymer,the coating being present in an amount to provide a yield of chlorinedioxide of at least 25 wt % at a conversion of chlorite anion tochlorine dioxide of at least 70 wt % when the composition is contactedwith water, wherein the coating being constructed to provide agelatinous membrane encapsulating the chlorite donor upon contact withwater which slows a dissolution rate of the chlorite donor, creates achamber in which chlorine dioxide is produced, restricts diffusion ofthe chlorite donor and chlorine dioxide out of the chamber, andrestricts diffusion of water into the chamber, and all wt % being basedon the total weight of the composition unless otherwise stated.
 2. Thecomposition according to claim 1, wherein the coating is constructed todissipate when a depletion level is reached inside the chamber.
 3. Thecomposition according to claim 1, wherein the gel-forming polymer ispresent in an amount of 0.1 to 30 wt %.
 4. The composition according toclaim 1, wherein the gel-forming polymer is present in an amount of 0.5to 20 wt %.
 5. The composition according to claim 1, wherein all of thechlorite donor is coated.
 6. The composition according to claim 1,wherein the chlorite donor comprises sodium chlorite.
 7. The compositionaccording to claim 1, wherein the chlorite donor and the coating arepresent in an amount to provide at least 30 wt % yield of chlorinedioxide when the composition is contacted with water.
 8. The compositionaccording to claim 1, wherein the free halogen donor comprisestrichloroisocyanuric acid.
 9. A tablet composition according to claim 1,wherein the gel-forming polymer comprises polyvinyl alcohol.
 10. Thecomposition according to claim 1, wherein the gel-forming polymercomprises polyvinyl alcohol and the composition comprises a stiffeningagent comprising boron.
 11. The composition according to claim 1,wherein the free halogen donor is coated with a coating comprising atleast one gel-forming polymer.
 12. The composition of claim 1, furthercomprising a plurality of the tablets contained in a bulk package,wherein the tablets are in intimate contact with one another.
 13. Thecomposition of claim 1, further comprising a plurality of the tabletscontained in a multi-tablet chemical dispenser.
 14. The composition ofclaim 1, wherein the solid composition in the form of a tablet hasenhanced environmental stability.
 15. The composition of claim 1,wherein the solid composition in the form of a tablet provides acontrolled release of chlorine dioxide when contacted with water. 16.The composition according to claim 1, wherein the acid source comprisesat least one acid selected from the group consisting of dicarboxylicacids and tricarboxylic acids.
 17. The composition according to claim 1,wherein the acid source comprises fumaric acid.
 18. The compositionaccording to claim 1, wherein the acid source comprises tartaric acid.19. The composition according to claim 1, wherein the acid sourcecomprises citric acid.
 20. The composition according to claim 1, whereinthe acid source comprises succinic acid.
 21. The composition accordingto claim 1, wherein composition is self-limiting.
 22. The compositionaccording to claim 10, wherein the composition comprises an effectiveamount of combustion suppressing boron donor.
 23. The compositionaccording to claim 1, wherein the polymer comprises an oxidationresistance polymer to avoid reacting with the chlorite donor when amixture of the oxidation resistant polymer and chlorite donor is heatedto temperatures to at least 100° C.
 24. The composition according toclaim 1, wherein the at least the chlorite donor is coated with ananti-caking agent comprising magnesium carbonate.
 25. The compositionaccording to claim 1, wherein the at least the chlorite donor is coatedwith an anti-caking agent comprising fumed silica.
 26. A solidcomposition that produces chlorine dioxide on demand upon contact withwater, the composition comprising: a solid chlorite donor comprisingsodium chlorite in an amount to provide at least 25 wt % yield chlorinedioxide when the composition is contacted with water; a low solubilityfree halogen donor comprising trichloroisocyanuric acid in an amountfrom 12 to 60 wt % and in an amount to convert at least 70 wt % of thechlorite anion to chlorine dioxide when the composition is contactedwith water; fumaric acid in an amount to provide a pH of less than 7.8when 1 gram of tablet composition is dissolved in 100 ml of water; and acoating covering the chlorite donor, the coating comprising polyvinylalcohol, the coating being present in an amount to provide a yield ofchlorine dioxide of at least 25 wt % at a conversion of chlorite anionto chlorine dioxide of at least 70 wt % when the composition iscontacted with water, wherein the coating being constructed to provide agelatinous membrane encapsulating the chlorite donor upon contact withwater which slows a dissolution rate of the chlorite donor, creates achamber in which chlorine dioxide is produced, restricts diffusion ofthe chlorite donor and chlorine dioxide out of the chamber, and whereinall wt % being based on the total weight of the composition unlessotherwise stated.
 27. A method of producing chlorine dioxide on demandcomprising contacting a solid composition in the form of a tablet withwater, the solid composition comprising: at least one solid chloritedonor in an amount to provide sufficient chlorite anion to obtain atleast 25 wt % chlorine dioxide when the composition is contacted withwater; at least one solid form of free halogen donor in an amount from12 to 60 wt % and in an amount to convert at least 70 wt % of thechlorite anion to chlorine dioxide when the composition is contactedwith water; at least one solid acid source in an amount to provide a pHof less than 7.8 when 1 gram of tablet composition is dissolved in 100ml of water; and a coating covering at least the chlorite donor, thecoating comprising at least one gel-forming polymer, the coating beingpresent in an amount to provide a yield of chlorine dioxide of at least25 wt % at a conversion of chlorite anion to chlorine dioxide of atleast 70 wt % when the composition is contacted with water, wherein thecoating being constructed to provide a gelatinous membrane encapsulatingthe chlorite donor upon contact with water which slows a dissolutionrate of the chlorite donor, creates a chamber in which chlorine dioxideis produced, restricts diffusion of the chlorite donor and chlorinedioxide out of the chamber, and restricts diffusion of water into thechamber, and wherein all wt % being based on the total weight of thecomposition unless otherwise stated.
 28. The method according to claim27, wherein the coating dissipates when a depletion level is reachedinside the chamber.
 29. The method according to claim 27, wherein thegel-forming polymer is present in an amount of 0.1 to 30 wt %.
 30. Themethod according to claim 27, wherein all of the chlorite donor iscoated.
 31. The method according to claim 27, wherein the chlorite donorcomprises sodium chlorite.
 32. The method according to claim 27, whereinthe chlorite donor and the coating are present in an amount to provideat least 30 wt % yield of chlorine dioxide when the composition iscontacted with water.
 33. The method according to claim 27, wherein thefree halogen donor comprises trichloroisocyanuric acid.
 34. The methodaccording to claim 27, wherein the free halogen donor comprisesdichloroisocyanuric acid.
 35. The composition according to claim 27,wherein the free halogen donor comprises a low solubility free halogendonor having a solubility of no greater than 5 grams per 100 ml of waterat 25° C.
 36. The method according to claim 27, wherein the gel-formingpolymer comprises polyvinyl alcohol.
 37. The method according to claim27, wherein the gel-forming polymer comprises polyvinyl alcohol and thecomposition further comprises a stiffening agent comprising boron. 38.The method according to claim 27, wherein the free halogen donor iscoated with a coating comprising at least one gel-forming polymer. 39.The method according to claim 27 further comprising a plurality oftablets packaged in bulk wherein the tablets are in intimate contactwithin the package.
 40. The method according to claim 27 furthercomprising decanting the plurality of the tablets from a bulk packageinto a multi-tablet chemical dispenser prior to contacting the tabletswith the water.
 41. The method according to claim 27 further comprisingcontacting the tablets with the water in the multi-tablet chemicaldispenser, resulting in the controlled release of chlorine dioxide. 42.The method according to claim 27, wherein the acid source comprises atleast one acid selected from the group consisting of dicarboxylic acidsand tricarboxylic acids.
 43. The method according to claim 27, whereinthe acid source comprises fumaric acid.
 44. The method according toclaim 27, wherein the acid source comprises tartaric acid.
 45. Themethod according to claim 27, wherein the acid source comprises citricacid.
 46. The method according to claim 27, wherein the acid sourcecomprises succinic acid.
 47. The method according to claim 27, furthercomprising using the chlorine dioxide produced as an antimicrobialagent.
 48. The method according to claim 27, further comprising usingthe chlorine dioxide produced for the treatment of food processingapplications.
 49. The method according to claim 27, further comprisingusing the chlorine dioxide produced for the treatment of recirculatingsystems.
 50. The method according to claim 27, further comprising usingthe chlorine dioxide produced for the treatment of hard surfaces. 51.The method according to claim 27, further comprising using the chlorinedioxide produced for the treatment of emergency drinking water.
 52. Themethod according to claim 27, further comprising using the chlorinedioxide produced for the treatment of surgical instruments andequipment.
 53. The method according to claim 27, further comprisingadjusting the rate of formation of chlorine dioxide by adjusting theamount of coating present on the chlorite donor.
 54. The methodaccording to claim 27, wherein composition is self-limiting such thatwhen the concentration of chlorine dioxide and composition componentsreach a saturation level the generation of chlorine dioxide ceases. 55.The method according to claim 27, wherein the polymer has sufficientoxidation resistance to avoid reacting with the chlorite donor when amixture of oxidation resistant polymer and chlorite donor is heated totemperatures to at least 100° C.
 56. The method according to claim 37,wherein the composition comprises an effective amount of combustionsuppressing boron donor.
 57. The method according to claim 27, whereinthe at least the chlorite donor is coated with an anti-caking agentcomprising magnesium carbonate.
 58. The method according to claim 27,wherein the at least the chlorite donor is coated with an anti-cakingagent comprising fumed silica.